In general, couplings for ducts of this type comprise a metal bushing which is fixed, e.g. crimped, on the end of the duct, and which is intended to engage on a tubular endpiece of an element of the above-mentioned circuit, the bushing ensuring that the duct and the endpiece are fluid-tight. A cylindrical locking member surrounds the bushing and the endpiece and co-operates with them by hooking and/or snap-fastening to oppose separation thereof and to maintain the connection between the duct and the endpiece.
This type of coupling must be reliable, while also being as simple and as low cost as possible. It is also requested or desired that it be easy and quick to assemble and that it enable the duct and the endpiece to be disconnected by a quick movement that can be performed by one hand without a tool.